1. Field of the Invention
The present invention relates to a novel liquid crystal compound and a liquid crystal composition. More specifically, it relates to a liquid crystal compound having a negative dielectric anisotropy (Δ∈), a liquid crystal composition containing the compound, and a liquid crystal display device containing the liquid crystal composition.
2. Related Art
A liquid crystal display device is classified, depending on the display mode thereof, into such modes as DS (dynamic scattering), TN (twisted nematic), GH (guest host), STN (super twisted nematic), IPS (in-plane switching), VA (vertical alignment) and OCB (optically compensated bend). A liquid crystal composition contained in the liquid crystal display devices desirably has some or all of the following common characteristics in all the display modes.    (1) The composition is stable to external environmental factors, such as water, air, heat and light.    (2) The composition exhibits a liquid crystal phase in a wide temperature range centering around room temperature.    (3) The composition has a small viscosity.    (4) The composition can decrease a driving voltage upon driving the display device.    (5) The composition has an optimum dielectric anisotropy (Δ∈).    (6) The composition has an optimum refractive index anisotropy (Δn).
However, such a liquid crystal compound has not yet been found that satisfies all of characteristics (1) to (6) by a sole compound. Therefore, a liquid crystal composition is often obtained by mixing several kinds or a couple dozen kinds of liquid crystal compounds. Accordingly, the liquid crystal compounds used as components of the composition necessarily have good compatibility with each other. A liquid crystal display device capable of being used in various environments, such as a very low temperature, has been demanded in recent years, and liquid crystal compounds exhibiting good compatibility at a very low temperature are thus also demanded.
In recent years, among the aforementioned display modes, such modes as IPS, VA and OCB are receiving attention as a display mode capable of overcoming a narrow viewing angle of a liquid crystal display device, which is the biggest problem of a liquid crystal display device. In particular, a liquid crystal display device of the VA mode and the IPS mode among these modes is being developed earnestly since it has excellent responsivity in addition to the wide viewing angle, and is capable of providing high-contrast display. The characteristics of the liquid crystal composition used in the liquid crystal display device of these modes reside in a negative dielectric anisotropy (Δ∈). It has been known that a liquid crystal composition having a negatively large dielectric anisotropy (Δ∈) can decrease the driving voltage of a liquid crystal display device containing the liquid crystal composition (as described in M. F. Leslie, Mol. Cryst. Liq. Cryst., vol. 12, p. 57 (1970)). Accordingly, liquid crystal compounds as the constitutional components of the liquid crystal composition are also demanded to have a negatively larger dielectric anisotropy (Δ∈).
As a component of a liquid crystal composition having a negative dielectric anisotropy (Δ∈), various liquid crystal compounds where hydrogen at a lateral position of a benzene ring is replaced by fluorine have been investigated (as described, for example, in Japanese Patent No. 2,811,342 and JP H2-4725 A/1990). The following compound (a) has been reported, for example. However, the compound (a) has a negative dielectric anisotropy (Δ∈), but the extent thereof is not necessarily large in some cases, and therefore, the compound is not sufficient in some cases for decreasing the driving voltage of a liquid crystal display device of the VA mode and the IPS mode.
wherein R and R′ each are alkyl.
Under the circumstances, there have been made attempts to increase the absolute value of the negative dielectric anisotropy (Δ∈) of the compound having a 2,3-difluorophenylene skeleton. For example, such a compound has been reported that is obtained by introducing a tetrahydropyran-2,5-diyl skeleton to the compound having a 2,3-difluorophenylene skeleton (as described, for example, in JP 2000-8040 A). The compound (b) has a negatively larger dielectric anisotropy (Δ∈) than the compound (a).

However, for decreasing the driving voltage of a liquid crystal display device of the VA mode and the IPS mode, a liquid crystal compound that has a negatively larger dielectric anisotropy (Δ∈), a liquid crystal composition containing the compound, and a liquid crystal display device containing the composition have been demanded.